Humidity sensor

ABSTRACT

A humidity sensing portion (7) and an electrode (9) of a heating regeneration type humidity sensor provided with a heater (8) are covered by a casing (12) and a holding portion (11). An opening (6) for communication between placing the interior and exterior of the sensor in communication is formed in the casing (12) or the holding portion (11) so that a solid angle, of a vertex at the central portion of the humidity sensing portion (7) or the casing (12) of a polyhedron having a base bounded by the opening is within the range of 1/1000-1/10 radian, and thus, adhesion of harmful substances, especially inorganic harmful substances, onto the humidity sensing portion (7) or the electrode (9) is prevented.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a humidity sensor employed formeasuring humidity in the atmosphere or humidity control.

2. Description of the Related Art

Conventionally, for the measurement of humidity, there have beememployed a hair hygrometer, wet and dry bulb hydrometers or the like.However, due to the recent progress in the field of electronictechniques, electrical measurements are brought into application due tosimplicity in operation. In the electrical measurements referred toabove, materials whose resistance values or dielectric constants varydepending on humidity are employed to convert the humidity variationsinto quantities of electricity due to variations in the resistancevalues or dielectric constants. By way of example, as the materialswhose resistance values are altered, metallic oxides or ceramics areemployed, while for the materials whose dielectric constants are varied,organic films may be used. However, since these humidity sensors arearranged to detect the humidity through contact thereof with theatmosphere, there has been a disadvantage in that the characteristics ofthe humidity sensors are deteriorated by adhesion of atmosphericpollutants to the surfaces of the humidity sensors.

Therefore, it has been a recent practice to prevent the deterioration ofthe humidity sensor characteristics by a heating regeneration method inwhich harmful substances adhering to the surface of the humidity sensorare removed by heating the sensor at high temperatures or by a method inwhich the humidity sensor is covered by a surface soiling preventionfilter. Such a surface soiling prevention filter is mainly applied to anorganic film type sensor which can not be subjected to the heatingregeneration, while the heating regeneration method is utilized for thehumidity sensor employing ceramics.

The humidity sensor to which such heating regeneration is applied willbe described hereinbelow with reference to FIG. 10.

In FIG. 10, an electrode 3 is fixed on a surface of a humidity sensingportion 1 composed of ceramics and the like, while a heater 2 isattached to its surface confronting this electrode 3. The humiditysensing portion 1, heater 2 and electrode 3 are fixed to a holdingportion 4, and a protector 5 enclosing these parts for protectionthereof against mechanical damages is secured to the holding portion 4.This protector 5 is formed with a large number of through-holes 5a inits peripheral face for improved heat radiation. Moreover, lead wires 6electrically connectioned with the electrode 3 are connected with theelectrode 3 through the holding portion 4.

When the humidity sensor having the construction as described above isbrought into contact with the measuring atmosphere, water vapor in theatmosphere arrives at the humidity sensing portion 1 by diffusion or byair stream so as to be absorbed thereonto, and depending on the amountof absorption at that time, the humidity sensing portion 1 undergoes avariation in its electrical characteristics. If the variation ispreliminarily applied in the form of voltage to the electrode 3 throughthe lead wires 6, humidity in the atmosphere may be electricallydetected. Meanwhile, when the characteristics of the humidity sensor aredeteriorated by the adhesion of harmful substances onto the surface ofthe humidity sensing portion 1, it has been a practice to restore thecharacteristics by heating the humidity sensing portion 1 throughenergization of the heater 2 so as to burn the adhering harmfulsubstances into the form of a gas for removal.

However, in the conventional humidity sensor, harmful substances arealso drawn in together with the atmosphere via the through-holes 5a ofthe protector 5 to adhere to the humidity sensing portion 1. Suchharmful substances may be broadly divided into the harmful substances oforganic nature such as soot, smoke of cigarettes, etc. and also intothose of inorganic nature such as salt particles, dust, etc. Althoughremoval of organic harmful substances can be effectively made by theheater 2, such heating is hardly effective for the inorganic harmfulsubstances such as salt particles, dust and the like which can not beformed into a gas by combustion for removal. Moreover, since the heater2 is subjected to high temperatures above 500° C., the protectorcovering the heater 2 is also heated up to a high temperature, and ifany combustible material is located close to the protector 5, there issuch a danger that the combustible material is ignited. Furthermore,there is also such a problem in that, if the humidity sensor is providedin a combustible gas atmosphere, an explosion may take place by the heatof the heater 2.

SUMMARY OF INVENTION

Therefore, according to the present invention, the humidity sensingmaterial is enclosed by a holding portion and a casing, with an openingbeing provided in said holding portion or casing for placing theinterior and exterior of the holding portion in communication, and saidopening has a solid angle in the range of 1/1000 to 1/10 of a radian,i.e. an opening defining a boundary of the base of a polyhedron andsubtending a solid angle between 1/1000 and 1/10 of a radian at a vertexof the polyhedron, the vertex being located at the central portion ofthe humidity sensing portion or the casing, to prevent entry of theharmful substance detrimental to said humidity sensing portion forpreventing the deterioration thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a longitudinal cross-sectional view partly broken away,showing a humidity sensor according to one preferred embodiment of thepresent invention, FIG. 1(b) is a lateral cross-section of the sensor ofFIG. 1(a), FIG. 2 is a graph showing the relation between solid anglesof the opening and deterioration of the element when sea water issprayed onto the humidity sensor, FIG. 3(a) is a longitudinalcross-section artly broken away, showing a humidity sensor according toa second embodiment of the present invention, FIG. 3(b) is a bottom planview of the sensor of FIG. 3(a), FIGS. 4(a), 4(b) and 4(c) respectivelyshow side elevational views of humidity sensors according to third,fourth and fifth embodiments of the present invention, FIG. 5 is alongitudinal cross-sectional view partly broken away, showing a humiditysensor according to a sixth embodiment of the present invention, 4FIG.6(a) is a longitudinal cross section partly broken away, showing ahumidity sensor according to a seventh embodiment of the presentinvention, FIG. 6(b) is a lateral cross-sectional view of the sensor ofFIG. 6(a), FIG. 7 is a graph showing the relation between solid anglesof the opening and deterioration of the element when sea water issprayed onto the humidity sensor of FIG. 6, FIG. 8 is a graph showingthe relation between a diameter Dφ of a cylinder with a slit for thehumidity sensor in FIG. 6 and surface saturated temperature duringcontinuous heating thereof, FIGS. 9(a) and 9(b) are side elevationalviews showing other embodiments of the humidity sensors provided withthe second casing of the present invention, FIG. 10 (a) is alongitudinal cross-sectional view partly broken away, showing aconventional humnidity sensor of a heating cleaning type, and FIG. 10(b)is a lateral cross-sectional view of the sensor of FIG. 10(a).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to FIGS. 1 through 8.

FIGS. 1(a) and 1(b) show a humidity sensor according to a firstembodiment of the present invention. In FIGS. 1(a) and 1(b), numeral 7denotes a humidity sensing portion for detecting the humidity, numeral 8denotes a heater for cleaning by heating the humidity sensing portion 7,and numeral 9 denotes an electrode for converting the humidity intoelectrical signals. The humidity sensing portion 7, heater 8 andelectrode 9 are accommodated in a cylindrical casing 10 a bottom, withthe casing 10 being closed by a disc-like holding portion 11 to whichthe heater 8 is mounted. This casing 10 is formed with a slit-likeopening 12. To the holding portion 11, the humidity sensing portion 7and the heater 8 are fixed. The opening 12 is intended to allow thehumidity sensing portion 7 to contact the atmosphere. This opening 12 islocated below the humidity sensing portion 7 and the electrode 9subtends a solid angle of a vertex of a polyhedron, the vertex at thecentral portion of the humidity sensing portion 7 and/or casing 10 andthe base of the polyhedron defined by the opening 12, of less than 1/10of a radian. Numeral 13 denotes lead wires for voltage impression andsignal output, which extend from the electrode 9 through the holdingportion 11.

With respect to the humidity sensor having the construction as describedabove, the operation thereof will be described hereinbelow.

In the construction as described so far, if the opening 12 is providedat a predetermined position, so as to extend vertically downwardly, thehumidity sensing portion 7 and the electrode 9 are covered by the casing10 in the surrounding portion thereof, and are brought into contact withthe atmosphere only through the slit-like opening 12 located thereunderfor the detection of the humidity. With the above arrangement, since theinorganic solid particles such as salt particles, dust, etc. in theatmosphere can not move through the atmosphere by diffusion as in watervapor such particles do not adhere to the humidity sensing portion 7 andthe electrode 9 even if they enter through the opening 12 in a windlessstate, and freely drop so as to merely accumulate onto the upper portionof the casing 10 at the most. Moreover, even in the presence of wind,since the solid angle of the opening 12 is less than 1/10 of a radian,with said opening being provided only at one side with respect to thedirection of the wind, ventilation is not readily established.Meanwhile, even if wind should be directed from a vertically upwarddirection, the harmful particles do not readily adhere to the humiditysensing portion 7 and the electrode 9, since the opening 12 is locatedin a position lower than said humidity sensing portion 7 and electrode9. As described above, the humidity sensing portion 7 and the electrode9 can be protected against adhesion of the inorganic solid particles inany of the above states. FIG. 2 is a graph showing the relation betweenthe solid angles of the opening 12 and the deterioration of the sensorwhen sea water is sprayed with respect to the opening 12 of the humiditysensor in a directly lateral direction at a wind velocity of about 0.55m/s. As shown in FIG. 2, it is understood that, by setting the solidangle thereof smaller than 1/10, deterioration of the sensor by the saltcomponent in the presence of wind can be markedly reduced.

On the other hand, water vapor in the atmosphere is absorbed onto thehumidity sensing portion 7 through the opening 12 by diffusion even inthe windless state, and in that case, varies the electricalcharacteristics of the portion 7 according to the amount of absorption.The variation in the resistance of portion 7 is converted intoelectrical signals by the electrode 9 supplied with voltage through thelead wires 13. In the manner described above, the sensor is protectedagainst adhesion of inorganic solid particles in the atmosphere, anddetects the humidity in the atmosphere.

In FIGS. 3(a) and 3(b), there is shown a second embodiment according tothe present invention. Since the humidity sensing portion 7, heater 8,electrode 9, holding portion 11, and lead wires 13 are the same as thosein the first embodiment, like parts are designated by like referencenumerals, with detailed description thereof being omitted for brevity.The difference thereof from the first embodiment resides in an opening14. More specifically, the opening 14 is provided not in the casing 15,but in the holding portion 11. By this construction, the humiditysensing portion 7 and the electrode 9 are completely covered by thecasing 15 without any opening so as not to be affected to a substantialdegree by the state of a surrounding air stream so as to be protectedagainst the adhesion of inorganic solid particles. Moreover, if theabove opening 14 is commonly used with the through-opening for the leadwires 13, it is advantageous from the viewpoint of the manufacture ofthe humidity sensor.

It is to be noted here that, if the solid angle of each opening islimited within the range of 1/10-1/1000, the position, configuration,number, and state of the of openings may be modified as shown in FIGS. 4and 5. More specifically, in FIG. 4(a), there is shown a thirdembodiment in which a plurality of openings 16a are formed on the sideface of the casing 16 in two rows in a longitudinal direction. FIG. 4(b)shows a fourth embodiment in which openings 17a are formed on the casing17 in one row in a longitudinal direction. FIG. 4(c) shows a fifthembodiment in which openings 18a are formed on the side face of thecasing 18 in the form of a cross. Furthermore, FIG. 5 shows a sixthembodiment in which an opening 19a is formed on the upper surface of thecasing 19.

Additionally, if the width of the opening in the embodiment as describedso far is set to be smaller than 1 mm, the casing may serve as anexplosion-proof structure. In other words, even when a flammablesubstance is present around the sensor during the cleaning by heating orthe casing, catches on fire, the fire does not spread towards theoutside of the casing due to the small opening width, and thus, a dangerof explosion may be prevented.

Referring now to FIG. 6, a seventh embodiment according to the presentinvention will be described hereinbelow. In FIG. 6, numeral 20 denotes ahumidity sensing portion for detecting the humidity, numeral 21 denotesa heater for cleaning the humidity sensing portion, and numeral 22denotes an electrode for converting the humidity into electricalsignals. The humidity sensing portion 20, heater 21 and electrode 22 areaccommodated in a first casing 23 having a cylindrical configurationwith a bottom, and said first casing 23 is closed by a columnar holdingportion 24 to which the heater 21 is attached. This first casing 23 has,on its side face, a slit-like opening 26 extending in a longitudinaldirection so as to allow the humidity sensing portion 20 to directlycontact the atmosphere. The first casing 23 is enclosed in a secondcasing 25 which is closed at the bottom portion by the holding portion24, and at the upper portion by a cap 25a, with the side wall beingcomprised a wire netting. Thus, this second casing 25 is provided with alarge number of through-holes 27 each having a width of loss than 1 mm.To the holding portion 24, the humidity sensing portion 20 and theheater 21 are fixed. Lead wires 28 for voltage impression and signaloutput extend form the electrode 22, and extend through the holdingportion 24.

In the above-described embodiment, when the sensor is positioned at thepredetermined position, with the opening 26 directed verticallydownwardly, the humidity sensing portion 20 and the electrode 22 whoseperipheral portions are covered by the first and second casings, contactthe atmosphere through the through-holes 27 and the opening 26 to detectthe humidity. With the above arrangement, the inorganic solid particlessuch as salt particles, dust, etc. in the atmosphere, which can not moveby diffusion as in water vapor, freely fall in a windless state so as toaccumulate on the upper surface of the first and second casings, and donot adhere to the humidity sensing portion 20 and the electrode 22 byentering through the opening 26. Meanwhile, in the presence of windalso, since the opening 26 has a solid angle of less than 1/10 of aradian, and is formed only at one side with respect to the direction ofwind so as to prevent an air stream from passing through the sensor, andsince said opening 26 is provided below the humidity sensing portion 20and the electrode 22, harmful particles do not readily adhere to thesensing portion 20 and the electrode 22, even if the wind should blowvertically upwardly. As described above, the humidity sensing portion 20and the electrode 22 remain substantially free from the adhesion ofinorganic solid particles in any of the aforementioned states. FIG. 7 isa diagram showing the relationship between the solid angles of theopening 26 for the first casing and the amount of deterioration of thesensor in the case where sea water is sprayed with respect to theopening 7 from a directly lateral direction at a wind velocity of about0.55 m/s. As is seen from the diagram, by setting the solid angle atless than 1/10 of a radian, the deterioration of the sensor by the saltparticles is markedly reduced even when a wind blows in a lateraldirection. Incidentally, when the sensor is deteriorated by the adhesionof organic substances such as tobacco smoke, oil vapor, etc. onto thehumidity sensing portion 20 and the electrode 22, it is possible torestore the deteriorated characteristics by removing said organicsubstances through heating by means of the heater 21. In this case,however, there may a situation in which the surface of the first casing23 is excessively heated to about 100° C., with the heater being leftenergized for a long period of time by some causes. In such a case,since the first casing 23 is covered by the second casing 25, thesurface temperature of the second casing 25 can be suppressed to be lowas compressed with that of the first casing 23, and there is no dangerof fire even if a flammable material such as paper or the like isbrought into contact with the second casing 25.

FIG. 8 is a graph showing the relation between cylinder surfacesaturated temperatures and diameters Dφ of cylinders in which aslit-like opening having a width of 1 mm and a length of 30 mm is formedon the side face of a tightly closed cylinder diameter Dφ mm and heightof 30 mm, with a heater of 4.5 W being provided in said cylinder forcontinuous heating under the atmosphere at room temperature of 25° C.,and a wind velocity of less than 0.05 m/s. If the diameter Dφ is largerthan 20 mm, it becomes possible to reduce the surface temperature lowerthan 70° C. Accordingly, in the case of cleaning by heating, even whenthe surface temperature of the first casing is raised above 100° C. bysome causes, the surface temperature of the sensor may be limited to beless than 70° C., since the second casing 25 having diameter of about 20mm and having the large number of through-holes on the side face coverssaid first casing 23.

As is seen from the foregoing description, according to the humiditysensor of this embodiment, it is possible to detect the humidity in theatmosphere without subjecting the sensor to the problems associated withadhesion of organic solid particles in the atmosphere, and with thesensor outer covering temperature being limited to be below 70° C. atall times during cleaning by heating for safety.

It is to be noted here that in the above seventh embodiment, althoughthe through-holes 27 are provided on the side face of the second casing25, it may be so modified that a large number of slits 29a are providedon the side face of the second casing 29 as shown in FIG. 9(a).Moreover, as shown in FIG. 9(b), the second casing 30 itself maycomprise a net-like form to provide the through-holes 30(b). By usingthe net-like structure as described above, even when a flammablesubstance catches fire from the heater in the first casing, spreading offlames outwardly from the second casing 30 is more effectivelyprevented, thereby to positively improve the explosion-proof property ofthe sensor.

Furthermore, by forming the sizes of the opening and through-holes inthe foregoing embodiment to be less than 1 mm, the explosion proofproperty is imparted to the first and second casings, and since thesensor as a whole is thus provided with a double explosion proofconstruction, danger of explosion may be avoided even if any flammablesubstance is present in the surroundings during cleaning by heating.

As described so far, according to the present invention, due to thearrangement that the opening for placing the interior and exterior ofthe sensor in communication is formed in the casing enclosing, thehumidity sensing portion and the heating portion, or in the holdingportion which supports the humidity sensing portion and the heatingportion to close the casing, with said opening having a solid angle fromthe central portion of the humidity sensing portion or the casing in therange of 1/1000-1/10, entry of harmful substances, especially inorganicsubstances incapable of being regenerated by heat, through the openingcan be suppressed, and thus, deterioration of the humidity sensor may bereduced as far as is practicable to prolong the life of the sensor.Accordingly, a humidity sensor, which normally functions for a longperiod even when used in circumstances full of harmful substances, maybe provided, and thus, the range of application of the humidity sensorcan be markedly widened.

What is claimed is:
 1. A humidity sensor comprising:a humidity sensingportion having an electrode; a heating portion for heating said humiditysensing portion; a holding portion to which the humidity sensing portionand the heating portion are mounted; and a casing for enclosing thehumidity sensing portion and the heating portion mounted to the holdingportion, at least one of said casing and said holding portion having anopening extending therethrough for placing space interior of the casingin communication with the ambient temperature exterior of the casing,said opening defining the base of a polyhedron having a vortex at thecenter of the casing, the solid angle of the polyhedron at said vertexbeing between 1/100 and 1/10 of a radian for inhibiting inorganicsubstances from entering the interior space of the casing from theatmosphere.
 2. A humidity sensor as claimed in claim 1,wherein saidcasing has a double-walled construction comprising a first inner casingthrough which said opening extends and a second casing extending aroundsaid first casing and through which a plurality of through-holes extend.3. A humidity sensor as claimed in claim 1 or claim 2,wherein theopening has a width that is less than 1 mm.
 4. A humidity sensor asclaimed in claim 1 or claim 2,wherein said opening is a slit.
 5. Ahumidity sensor as claimed in claim 1 or claim 2,and further comprisingat least one other opening extending through the casing, said otheropening extending through the casing at a location thereon relative tothe location on the casing through which said opening extends whichinhibits an air stream originating in the atmosphere from passingthrough said casing.
 6. A humidity sensor as claimed in claim 2,whereinsaid second casing comprises a net-like structure.